The present invention relates to a semiconductor light-emitting device composed of a Group III-V nitride semiconductor which is capable of outputting light ranging in color from blue to ultraviolet and to a method for fabricating the same.
In recent years, semiconductor light-emitting devices each using a Group III-V nitride semiconductor represented by a general formula: BxAlyGa1-x-y-zInzN (where x, y, and z satisfy 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, x+y+z=1), i.e., a light-emitting diode device and a semiconductor laser device have been developed vigorously as light sources for emitting light ranging in color from blue to ultraviolet.
Referring to the drawings, a conventional semiconductor light-emitting device composed of a Group III-V nitride semiconductor will be described.
As shown in FIG. 13, an n-type contact layer 102 composed of n-type GaN, an n-type cladding layer 103 composed of n-type AlGaN, an active layer 104 composed of GaInN, a p-type cladding layer 105 composed of p-type AlGaN, and a p-type contact layer 106 composed of p-type GaN are formed successively on a substrate 101 composed of, e.g., sapphire by epitaxial growth.
A current blocking layer 107 composed of a silicon dioxide or a silicon nitride and having an opening 107a for current confinement is formed on the p-type contact layer 106. A p-side electrode 108 is formed on the portion of the p-type contact layer 106 exposed through the opening 107a of the current blocking layer 107.
As another method involving the provision of a current confining structure, there has been known one which confines a current path by removing, from a laser device structure, at least the both side portions of the p-type cladding layer 105 by etching.
In the conventional semiconductor light-emitting device, however, a silicon dioxide or silicon nitride is deposited by chemical vapor deposition or the like to form the current blocking layer 107 on the p-type contact layer 106. Each of the silicon dioxide and silicon nitride has the problems of poor adhesion to a group III-V nitride semiconductor, a high density of small holes, i.e., a high pinhole density, and the like.
If the current confining structure is formed by removing the both side portions of the cladding layer by etching, the electrode should be formed on the top surface of the ridge region (mesa region) formed by the etching process so that the area of the electrode is reduced. This causes the problem of an increased DC resistance component in a current path.
If the conventional semiconductor light-emitting device is a semiconductor laser device, recombined light generated in the active layer 104 is confined by the current blocking layer 107 composed of a dielectric material to the inside of the Group III-V nitride semiconductor due to a refractive index difference between the active layer 104 and the semiconductor. Since the refractive index difference is relatively large and varies discontinuously (stepwise), if the recombined light is to be confined in, e.g., a single lateral mode, the width of the opening 107a (stripe width) of the current blocking layer 107 is reduced excessively so that it becomes difficult to optimize the laser structure. If the stripe width is reduced excessively, the DC resistance component is increased disadvantageously as described above.
In addition, though not shown in the drawings, there are many cases observed where a conventional semiconductor laser device uses a cavity having a ridge structure. In a case with the ridge structure, the efficiency of the light confinement depends on a difference between a first refractive index inside the ridge region and a second refractive index in a region other than the ridge region. In detail, the first refractive index means a first effective refractive index determined according to each refractive index and each thickness of the semiconductor layer composing the active layer and of the semiconductor layer composing the cladding layer, and the second refractive index means a second effective refractive index determined according to each refractive index and each thickness of the semiconductor layer composing the active layer, the semiconductor layer composing the cladding layer and, for example, a silicon oxide layer or a silicon nitride layer composing the sides of the ridge structure. In the conventional ridge structure, the difference between the first refractive index and the second refractive index varies discontinuously (stepwise) and the step difference is rather large. Because of the large confinement efficiency, the light emitting point of the laser light may displace at a high power output and the configuration of the spot is liable to change when the light is confined in the cavity under this condition. For this reason, the design for optimizing the laser structure is rather difficult in equipment requiring accurate control of the light emitting point and the spot configuration, such as an optical laser disk device.
It is therefore an object of the present invention to solve the conventional problems and thereby improve the adhesion of a current blocking layer provided in a semiconductor light-emitting device composed of a Group III-V nitride semiconductor, while reducing a pinhole density therein.
To attain the object, a semiconductor light-emitting device according to the present invention comprises: a semiconductor cladding layer on a substrate; an active layer formed on the cladding layer; a semiconductor cladding layer formed on the active layer, the semiconductor cladding layer being composed of a Group III-V nitride; and a current blocking layer formed in the semiconductor layer to have an opening for exposing the semiconductor layer therethrough, the current blocking layer being composed of a dielectric material obtained by replacing some of nitrogen atoms composing the semiconductor layer with oxygen atoms.
In the semiconductor light-emitting device according to the present invention, the dielectric material obtained by replacing some of the nitrogen atoms composing the semiconductor layer composed of the group III-V nitride is used for the current blocking layer so that the current blocking layer is formed integrally with the semiconductor layer. This resolves the problem of the current blocking layer associated with the adhesion thereof to the semiconductor layer and significantly reduces the pinhole density.
In the semiconductor light-emitting device according to the present invention, a composition of oxygen in the current blocking layer preferably decreases gradually with approach to the active layer.
In the arrangement, the composition of oxygen in the semiconductor layer increases gradually with distance from the inside of the semiconductor layer toward the outside thereof so that the refractive index difference between the semiconductor layer and the current blocking layer changes continuously. This allows an increase in the width of an opening in the current blocking layer for effecting single lateral mode confinement if the semiconductor light-emitting device is, e.g., a laser device and permits easy optimization of the laser structure.
In the semiconductor light-emitting device according to the present invention, the opening of the current blocking layer preferably has a stripe plan configuration.
In that case, the semiconductor layer preferably has a ridge portion composing a cavity and the current blocking layer is formed also on side portions of the semiconductor layer.
In the semiconductor light-emitting device according to the present invention, the opening of the current blocking layer preferably has a dot plan configuration.
A method for fabricating a semiconductor light-emitting device according to the present invention comprises: a first step of forming an active layer and a cladding layer on a substrate; a second step of forming, on the active layer, a semiconductor layer composed of a Group III-V nitride; a third step of selectively forming, on the semiconductor layer, a mask film for partially masking the semiconductor layer; a fourth step of oxidizing the semiconductor layer with the mask film formed thereon in an oxidizing atmosphere to form, in the semiconductor layer, a current blocking layer composed of a dielectric material obtained by replacing some of nitrogen atoms composing the semiconductor layer with oxygen atoms; and a fifth step of removing the mask film.
In accordance with the method for fabricating a semiconductor light-emitting device according to the present invention, the semiconductor layer composed of a Group III-V nitride is formed on the active layer and the mask film is formed selectively thereon. The semiconductor layer is then oxidized by using the formed mask film to form the current blocking layer composed of a dielectric material obtained by replacing some of the nitrogen atoms composing the semiconductor layer with oxygen atoms. This prevents a problem associated with the adhesion between the current blocking layer and the semiconductor layer. Since some of the nitrogen atoms composing the semiconductor layer have been replaced with oxygen atoms which are larger in atomic radius than the nitrogen atoms, the volume of the semiconductor layer is increased so that a pinhole density in the current blocking layer is reduced significantly.
Preferably, the method for fabricating a semiconductor light-emitting device further comprises, between the third and fourth steps: a sixth step of patterning the semiconductor layer into a ridge configuration by using the mask.